Characterization of shale pore system: A case study of Paleogene Xin'gouzui Formation in the Jianghan basin, China

Abstract

The pore size classification (micropore <2 nm, mesopore 2–50 nm and macropore >50 nm) of IUPAC (1972) has been commonly used in chemical products and shale gas reservoirs; however, it may be insufficient for shale oil reservoirs. To establish a suitable pore size classification for shale oil reservoirs, the open pore systems of 142 Chinese shales (from Jianghan basin) were studied using mercury intrusion capillary pressure analyses. A quantitative evaluation method for I-micropores (0–25 nm in diameter), II-micropores (25–100 nm), mesopores (100–1000 nm) and macropores (>1000 nm) within shales was established from mercury intrusion curves. This method was verified using fractal geometry theory and argon-ion milling scanning electron microscopy images. Based on the combination of pore size distribution with permeability and average pore radius, six types (I-VI) shale open pore systems were analyzed. Moreover, six types open pore systems were graded as good, medium and poor reservoirs. The controlling factors of pore systems were also investigated according to shale compositions and scanning electron microscopy images. The results show that good reservoirs are composed of shales with type I, II and III pore systems characterized by dominant mesopores (mean 68.12 vol %), a few macropores (mean 7.20 vol %), large porosity (mean 16.83%), an average permeability of 0.823 mD and an average pore radius (ra) of 88 nm. Type IV pore system shales are medium reservoirs, which have a low oil reservoir potential due to the developed II-micropores (mean 57.67 vol %) and a few of mesopores (mean 20.19 vol %). Poor reservoirs (composed of type V and VI pore systems) are inadequate reservoirs for shale oil due to the high percentage of I-micropores (mean 69.16 vol %), which is unfavorable for the flow of oil in shale. Pore size is controlled by shale compositions (including minerals and organic matter), and arrangement and morphology of mineral particles, resulting in the developments of shale pore systems. High content of siliceous mineral and dolomite with regular morphology are advantage for the development of macro- and mesopores, while high content of clay minerals results in a high content of micropores.